Technology stolen

[kabuto]

My nation has the most advanced technology in my current game compared to other nations. And for about 5 years now, it seems the whole continent uses spies to steal tech from me. I get the message that tech has been stolen about every 3-4 seasons. Now, I dont complain about them trying to steal from me. I think it makes sense that they do this but I dont know how I can prevent them from stealing.

I have 5 spies arranged around my capital and they all are lvl 6+ (might be 8+) so their influence area covers every possible approach to my capital. Yet I have never seen an enemy spy move towards or away from my capital. Is there any other way to make it harder for them to steal from me? Or am I doing something wrong?